The invention relates to a method and a system for optical detection of nano-objects in a refracting medium.
In the technical field of nano-sciences, in which the size of objects is an essential feature which characterizes them, techniques or methods for viewing these objects are fundamental.
The optical techniques used for this purpose at the present time are based on the phenomenon of luminescence. Fluorescent molecules have been studied and widely publicized, and are commonly used in the life sciences.
However, the aforesaid fluorescent molecules only allow short periods of observation, because of the phenomenon of photobleaching.
The development of more intense and stable luminescent objects has partially overcome the aforesaid problems, but only at the cost of the introduction of a considerable phenomenon of blinking.
Another method uses the absorption properties of the objects.
At the temperature of liquid helium, isolated molecules were initially detected by an absorption method based on the high quality factor of the zero phonon line, which has a very large absorption cross section of several 10−11 cm2 at resonance.
Isolated ions or atoms in rf (radio frequency) traps or high quality factor cavities have been detected by the absorption of a probe beam.
As a general rule, particles having a large absorption cross section and short time intervals between successive absorption events can be successfully detected by absorption methods.
More recently, a detection method for this type of particle has been developed for metallic particles. Metallic particles measuring about one nanometer, when excited in the vicinity of their plasmon resonance by an electromagnetic wave, have a relatively large absorption cross section, namely about 8×10−14 cm2 for a particle with a diameter of 5 nm, and a fast electron-photon relaxation time of about one picosecond.
Since the luminescence phenomenon of these particles is very weak, almost all of the absorbed electromagnetic energy is converted to heat. The resulting temperature rise causes a change in the local refractive index.
The aforesaid more recent method makes use of a method of interference contrast in polarized light, for detecting this local photothermal effect and displaying images of gold particles 5 nm in diameter have been obtained, with a signal to noise ratio of about 10.
For a more detailed description of the aforesaid method, reference may be made to the article entitled Photo thermal Imaging of Nanometer-Sized Metal Particles Among Scatterers, by David Boyer, Philippe Tamarat, Abdelhamid Maali, Brahim Lounis, and Michel Orrit, in: Science, Volume 297, 16 Aug. 2002, pp. 1160-1163, published by the American Association for the Advancement of Science.
The aforesaid method is satisfactory.
However, since it uses an interferometric method, it requires the provision of precision installations, particularly bulky and heavy optical benches, which are necessary for precision and for the preservation of the phase relations between the laser beams which is essential for distinguishing the interference phenomenon.
This type of installation appears inappropriate for the development of industrial or even laboratory installations which are easy to use.